Nuclear Reaction: How the story begins

Mohammad Din Alamin
Aug 9, 2017 · 4 min read

Nuclear energy has been one of the major prospects in the current industry of energy. The name itself, as it suggests, tells that it is a form of energy that is derived from something related to nucleus. This energy in general can be obtained from either splitting a nucleus or merging nucleus.

At the very beginning of time, after the big bang, only the lighter elements were produced. The others have been synthesized by nuclear reactions over times in stars and it takes large stars and supernovas to produce the heaviest elements.

WE ARE ALL STARDUST

So in the making of this form of energy question may arise inside our mind, what is the actual thing providing such kind of energy. The simple yet not so much known fact is that “Binding Energy” is the main thing behind this energy production.

Then what is this binding energy or the binding force?

Dr. Albert Einstein took over the issue from here. His introduction to ‘Special theory of relativity’ was mature enough to answer our unknowns in this particular. Previously it has been found out that there are deficiency of masses inside a nucleus from nucleus’s constituents individual summation of masses. As theory of relativity stated, mass and energy are not very much different things, E = mc² , and this lost mass is used in producing energy that helps in holding the nucleons together inside a nucleus. This energy is known as binding energy.

Well now how this binding energy define nuclear reaction?

Splitting of nucleus by neutrons, fission reaction, and merging lighter nucleus together, fusion reaction, provides huge energy. This is all due to this binding energy inside the nucleons. By splitting or merging nucleons inside the nucleus releases this energy, generally in kinetic form of energy.

What is the way of determining which will go through what kind of nuclear reaction?

Nuclei with a large average binding energy per nucleon are generally more stable.

For elements lighter than iron-56, fusion will produce large energy as nuclear binding energy increases with the increasing mass. Elements heavier than iron-56 will generally release energy upon fission as lighter product elements have more binding energy.

This peak in binding energy is due to the interplay between columbic repulsion and strong nuclear force.

This is due to the fact that, with the increase in size of nucleus strong nuclear force is felt only between closer nucleons. On the other hand columbic repulsion is felt throughout the nucleus. This phenomena leads towards instability and hence induces ‘radioactive’ and ‘fissile’ nature in the heavier elements.

Why fission needs neutrons?

According to classical physics, Alpha (α) particles can not escape from nucleus for columb barrier, which is a forbidden region and here kinetic energy is negative. But Quantum physics turns the way around. It states that this is possible through a process called “QUANTUM TUNNELING”. This is rather an advanced topic which I may not try to explain here.

So the basic thing is, for spontaneous fission of nucleus, the potential barrier has to be overcome by stretching big nucleus like a water droplets into a dumbbell shape which will eventually breaks apart and surpass this region of negative kinetic energy.

This work is done by neutron inside a reactor, it provides nucleus with extra binding energy what is needed to pass the barrier.

Can I carry out fission with particles other than neutron?

It is an interesting thing that only neutron can provide this binding energy to the nucleus for being electrically neutral particle.

Generally charged particles get interacted with the electron cloud(nothing to do with cloud computing) outside the nucleus of the atoms. So they can not get past this cloud to the nucleus. But neutron, being uncharged, easily gets past electrons and reaches towards the goal and ….

Fission occurs

Projectile ‘a’ + Nucleus ‘X’ → Nucleus ‘Y’ + ejectile ‘b’


There are some conservation laws for nuclear reactions: Conservation of charge and of atomic mass.

Fission process condition

Though strikes of neutrons start fission reaction not every strike considered potential fission starter. This is evaluated through Reaction Probability calculation. And also it has to be kept in mind that there are several reaction channels, like:

  1. Elastic Scattering
  2. Radiative Capture
  3. Fission
  4. Inelastic Scattering
  5. Other reactions

The probability cross section value for different reaction channels are different.

How to sustain fission energy?

Fission reaction is exothermic, produces around 200 MeV, that releases energy as a form of kinetic energy of the fission fragments and neutrons through emission of ‘gamma’ rays and decay of the fission fragments.

So if we can trigger a sustained chain event of reactions it may produce energy in a chained order and these energy’s accumulation will provide industrial level energy.

Future of fusion

Fusion is a great source of energy and can be considered as renewable form but it is sad that our technology still could not reach that level so that we can sustain this form of energy.


References:

  • Youtube video by: The New Jersey Center for Teaching and Learning
  • Lecture of Jan Heyse, Professor, Delft University of Technology
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